This invention relates to electro-optical devices whose electromagnetic radiation absorption characteristics can be selectively altered by influence of a suitably controlled electric field. More particularly, the invention is concerned with an electro-optical device which has a more uniform state of electrical charge at the imaging and/or counter-electrode areas. Still more particularly, this invention is directed to a sandwich type cell in which two layers of electrochromic material are separated by solid, semi-solid or liquid ion conducting media.
In U.S. Pat. Nos. 3,521,941, 3,578,843, 3,704,057 and 3,708,220, there are described electro-optical devices exhibiting a phenomenon known as persistent electrochromism wherein electromagnetic radiation absorption characteristic of a persistent electrochromic material is altered under the influence of an electric field. Such devices were employed in sandwich arrangement between two electrodes. Coloration was induced by charging the electrochromic film negative with respect to the counter-electrode, employing an external potential. The counter-electrode can be the same as the persistent electrochromic material or different.
By reversing the original polarity of the field or by applying a new field, it was also possible to cancel, erase or bleach the visible coloration.
The steps of color induction and erasure are defined as cycling.
It is an object of this invention to provide electrochromic imaging devices having more uniform operating characteristics.